1. Field of the Invention
The present invention relates to a lock mechanism for a webbing retractor which is employed in a seatbelt system for a vehicle designed to protect an occupant in an emergency situation of the vehicle, the lock mechanism being adapted for preventing an occupant restraining webbing from being unwound when an emergency situation of the vehicle occurs.
2. Description of the Related Art
In a typical seatbelt system for a vehicle, an occupant restraining webbing is wound up into a retractor from one end thereof by means of a predetermined biasing force, and the retractor incorporates a lock mechanism adapted for preventing the webbing from being unwound when an emergency situation of the vehicle occurs.
One type of conventional lock mechanism is arranged as follows. Namely, a lock ring is provided on a frame in such a manner as to follow the rotation of a webbing take-up shaft in a direction in which the webbing is unwound therefrom, with a resilient member interposed between the lock ring and the take-up shaft. When an emergency situation of the vehicle occurs, an acceleration sensing means acts to resist the rotation of the lock ring, so that a lock means which is adapted to rotate together with the take-up shaft is activated to rotate with a delay with respect to the rotation of the take-up shaft so as to engage with lock teeth provided on the frame, thus locking the webbing retractor (see the specifications of Japanese Patent Publication No. 16969/1969 and Japanese Patent Laid-Open No. 1115/1972).
This type of conventional lock mechanism suffers, however, the following problems.
When the vehicle is running on a rough road, the acceleration sensing means may be maintained in an operative state, and in such a case, the lock mechanism is held in the locking position, so that it is not possible for the occupant to unwind the webbing. In addition, when the vehicle is running on a rough road, the occupant's body is repeatedly moved up and down by vibrations of the vehicle, causing the webbing to be repeatedly wound in and out of the retractor. In consequence, as the webbing is wound up, the position at which a lock member of the lock means and the lock teeth engage with each other gradually progresses in a direction in which the webbing is wound up into the retractor, resulting in an increase in the pressure applied to the occupant's body from the webbing.
In order to overcome these disadvantages, the applicant of the present invention has already proposed a lock mechanism for a webbing retractor which has no risk of the webbing being gradually wound up into the retractor even when the webbing is repeatedly wound in and out by a given length (see the specification of Japanese Utility Model Application No. 003858/1985).
This proposed lock mechanism is arranged such that the rotation of a webbing take-up shaft is transmitted to a friction member through friction. Thus, when the take-up shaft rotates in a direction in which the webbing is wound up, the friction member is activated to cause a pawl lever to separate from a lock ring, thereby preventing the webbing from being wound up.
However, this friction member is allowed to turn only through an angle within a predetermioned range, and the take-up shaft alone rotates after the friction member has rotated a given angle. For this reason, a portion of the friction member which is brought into contact with the take-up shaft causes friction to resist the rotation of the take-up shaft. In order to cope with this resistance to the rotation of the take-up shaft, it is necessary to increase the biasing force of a spiral spring for winding up the webbing.